When exposed to radiation such as X-rays, an energy-storing phosphor (e.g., stimulable phosphor, which gives stimulated emission off) absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when it is exposed to a stimulating light. A radiation image recording and reproducing method utilizing the energy-storing phosphor has been widely employed in practice. In this method, a radiation image storage panel, which is a sheet comprising the energy-storing phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the storage panel; sequentially scanning the storage panel with a stimulating light such as a laser beam to produce stimulated emission; and photoelectrically detecting the emission to obtain electric image signals. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel is repeatedly employable.
The radiation image storage panel (often referred to as energy-storing phosphor sheet) has a basic structure comprising a support and an energy-storing phosphor layer provided thereon. If the phosphor layer is self-supporting, however, the support may be omitted. Further, a protective layer is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration and physical damage.
The phosphor layer generally comprises a binder and an energy-storing phosphor dispersed therein.
Since the radiation image recording and reproducing method has various advantages as described above, it is desired that the radiation image storage panel used in the method have a sensitivity as high as possible and give a reproduced radiation image of high quality (particularly in regard to sharpness and graininess).
Japanese Patent Provisional Publication 2000-284097 describes use of a radiation image storage panel employing tetradecahedral particles of stimulable rare earth metal-activated alkaline earth metal halide phosphor in which the particles have a mean particle size (Dm) in the range of 3.5 to 7.5 μm, a particle size distribution (Q) in the range of 0.500 to 0.800 so as to improve the image quality such as graininess. It is also described that the density of the phosphor particles in the phosphor layer can be increased by the use of phosphor particles comprising a mixture of phosphor particles having a particle size of 5.0 to 8.0 μm and phosphor particles having a particle size of 2.0 to 4.0 μ. Nevertheless, a density of the phosphor particles in the phosphor layer is increased up to a range of 3.12 to 3.32 g/cm3 according to working examples described in the Provisional Publication.
Japanese Patent Provisional Publication 9-269400 describes that the sensitivity and image quality can be improved by the use of a radiation image storage panel which has a phosphor layer containing 65% or more of phosphor particles. The phosphor particles comprise particles A having a size distribution peak in the range of 0.5 to 5 μm and particles B having a size distribution peak in the range of 6 to 30 μm in a ratio of 5:95 to 40:60. The size distribution peak of the particles B is larger than the size distribution peak of the particles A by 5 μm, or the former peak is as much as 3 times or more the latter peak.
Thus, it is known that the use of a mixture of plural groups of phosphor particles having different size distribution increases the density of phosphor particles in the phosphor layer and hence improves the image quality (such as graininess) of the resulting radiation image.